1. Field of the Invention
The present invention relates to an apparatus and method for a switched capacitor architecture for multi-band Doherty power amplifiers. More particularly, the present invention relates to an apparatus and method for a frequency tunable Doherty power amplifier having a switched capacitor architecture.
2. Description of the Related Art
The use of and consumer demand for mobile communications has continued to rapidly increase over the recent years, with the number of persons using mobile communications increasing while the number and types of data-rich mobile applications and services also rapidly increasing. Long Term Evolution (LTE) systems developed by the 3rd Generation Partnership Project (3GPP) are being implemented in order to satisfy the increased consumer demand. The LTE systems developed by the 3GPP provide a wireless communication network and mobile communication system having larger capacity, higher throughput, lower latency and better reliability. LTE systems utilize multiple Radio Frequency (RF) bands across a frequency spectrum that is defined for cellular and wireless applications. In particular, the LTE standard developed by the 3GPP defines over 27 different operational frequency bands.
FIG. 1 is a table illustrating LTE RF bands designated as Receive (RX) and Transmit (TX) RF bands according to the related art.
Referring to FIG. 1, typically, each of the RX and TX RF bands is approximately less than 80 MHz wide, however, the RX and TX RF bands are scattered across a frequency range of 698 MHz to 2680 MHz for base station RX bands that are used for uplink connections, and 728-2690 MHz for base station TX bands that are used for downlink connections. For example, within a frequency span of 365 MHz, there are four major RF bands defined in the LTE standard, including Digital Communication Service-1800 (DCS-1800), or band-3, Personal Communication Service (PCS1900), or band-2, Advanced Wireless Services (AWS), or band-4 and Universal Mobile Telecommunications Service 2100 (UMTS2100), or band-1.
For a mobile communication system having multiple RF bands, a transmitter of the related art includes dedicated Power Amplifiers (PAs) for each of the RF bands. Using dedicated PAs for each of the RF bands allows for narrow band matching networks that result in increased PA performance when compared to broadband PAs using wideband matching networks that are for more than one RF band of the multiple RF bands. Thus a number of PAs required in multi-band applications or mobile communication networks using an entirety of the LTE bands increases an amount of hardware included in the transmitter of the related art resulting in increasing costs and complexity with respect to network infrastructure and terminal applications.
A broadband PA can be used to operate across multiple RF bands but is prone to reduced gain, reduced power, reduced efficiency and reduced linearity as compared to a PA implementing narrowband matching. Accordingly, the broadband PA is not a suitable solution. A frequency tunable PA is a proposed solution to the problems of the broadband PA, however conventional PA technology does not include a suitable frequency tunable PA. Additionally, another important consideration in PA design is the power added efficiency of the PA. The leading PA efficiency enhancement techniques of the related art include supply modulation and load modulation techniques.
Supply modulation techniques, which include Envelope Tracking (ET), and Envelope Elimination and Restoration (EER), achieve high PA drain efficiency, for example 50-60%. Although EER can theoretically achieve higher efficiency than ET, EER suffers from a number of drawbacks such as low gain, stringent timing alignment, a high supply modulation bandwidth requirement, a high phase modulation path bandwidth, direct feed-through contamination at low output, and a low Power Supply Rejection Ratio (PSRR). For LTE and WiMax systems, envelope tracking PAs may have a final stage efficiency as high as 57% at 2.1 GHz using a High Voltage-Heterojunction Bipolar Transistor (HV-HBT). However, in order to achieve this, HV-HBTs have narrowband harmonic terminations within a packaging of the HV-HBT, and is not used for bandwidths greater than 100 MHz. Additionally, ET modulator signal bandwidth is limited to 20 MHz currently, which may increase up to 40 MHz, hence ET is not suitable for even moderate bandwidths of 60 MHz.
In contrast to supply modulation techniques, Doherty power amplifiers use a load modulation technique in order to achieve a high PA drain efficiency. Doherty PAs of the related art can achieve up to 57% average drain efficiency for LTE and WiMax signals. Doherty PAs are typically lower cost than the ET based PAs discussed above since they do not require an expensive envelope modulator, and therefore are more widely used in industry. However, Doherty PAs are considered to be narrow band due to the Doherty PAs' use of quarter-wave transformers resulting in narrow band operation of 60-100 MHz wide. Accordingly, neither envelope tracking PAs or Doherty PAs are suitable for wide bandwidth or multi-band applications
FIG. 2 illustrates a Doherty PA according to the related art.
Referring to FIG. 2, a Doherty PA 200 includes a signal input line 201 connected to a hybrid coupler 202. The hybrid coupler 202 splits an input signal received from the signal input line 201 so that the split input signal travels to both a first input matching network 203 and a second input matching network 204. The first and second input matching networks 203 and 204 convert a 50 ohm impedance of the signal input line 201 into a low impedance of a first amplifier 207, which is also referred to as a carrier transistor 207, and a second amplifier 208, which is also referred to as a peaking transistor 208, which are both connected to respective output matching networks 209 and 210 in order convert a low transistor output impedance into higher impedances for the Doherty output combiner 212. Offset lines 211 and 213 rotate an impedance of the peaking transistor 208 such that it appears close to be an open circuit at low drive levels so that the carrier transistor 207 does not waste power by driving signals into the output of the peaking transistor 208 which otherwise would degrade overall PA efficiency. The Doherty combiner is a 50 ohm λ/4 impedance inverter used to load modulate the transistors 207 and 208, and is used in conjunction with a 35 ohm load matching λ/4 transmission line in order to provide an approximately 50 ohm impedance to the output connector 215. Also shown in FIG. 2 are λ/4 gate and drain bias feed lines 206 for providing a high RF impedance to the carrier and peaking transistors 207 and 208 gate and drain nodes so that RF signals do not propagate into the DC power supply and spread RF into other circuits connected to the DC supply. Bias circuits 205 are used to convert a 5V DC supply voltage into a bias voltage that is a gate voltage to set the quiescent, or idle, current of the carrier and peaking transistors 207 and 208 in order to set a class of transistor operation. For example, the class of the transistor operation may be set to Class-AB for a carrier transistor and Class-C for a peaking transistor. The bias voltage may be positive or negative corresponding to a transistor technology.
In order to allow the Doherty PA 200 to be utilized for more than one band, a large number of the elements included in the conventional Doherty PA 200 shown in FIG. 2 are to be tuned. Specifically, the Doherty output combiner 212, the 35 ohm load matching λ/4 transmission line 214, the first and second input matching networks 203 and 204, and the first and second output matching networks 209 and 210, and the offset lines 211 and 213 are to be frequency tuned corresponding to each of the frequency bands in which the Doherty PA 200 operates. Additionally, the λ/4 bias feed lines 206 can also be tuned to each frequency band, although this is not required since not tuning these lines may only degrade overall PA efficiency by 1-2%.
Thus, the Doherty PA is frequency sensitive due to the various components of the Doherty PAs that are frequency dependent. Therefore, frequency tuning of the Doherty PA components is not easily done due to the lack of available high power tunable components. Accordingly, there is a need for a frequency tunable Doherty Power Amplifier in order to achieve high PA efficiency and linearity across multiple frequency bands, and hence, an apparatus and method for a switched capacitor architecture of a multi-band Doherty power amplification is needed.